1. Field of the Invention
The object of the present invention is a method of manufacturing a composite cord intented particularly, but not exclusively, for stringing tennis rackets. The invention is also directed to the cords obtained using this method.
2. Description of the Prior Art
It has long been known to manufacture cords from two or more components. This structure results from the fact that there are not available on the market fibers which combine the required tensile strength with the necessary hardness. When tension is applied, the fibers constituting the cord are brought into contact with one another and abrasion occurs leading to rapid wear. Also, the cords have to resist different forms of physical/chemical attack by virtue of their use in various environments. For this reason it has already been proposed to enclose the fibers constituting the cord in a protective sheath made of a material which is harder than the fibers themselves and which isolates them from attack by atmospheric agents. This action may be strengthened by internal lubrication.
The American patent U.S. Pat. No. 4,197,695 describes solution to the problem of protecting wires against corrosion, which consists in sealing individual strands with a plastic foam surrounded with a dense unfoamed plastic material The plastic foam coatings are sprayed on the strands and after dryed and cured by heating. Each strand embedded in the uncured foam plastic is then covered by a cover of unfoamed plastic material. However, this patent is not directed to the formation of a composite cord as in the present invention and it is necessary to provide a further heating to foam the first plastic material and to cure the unfoamed plastic material.
In the more particular case of tennis racket strings, the conditions of utilization are also very demanding. These strings must have elastic properties which are a priori incompatible with the tensions which players insist on. Moreover, their criss-cross application involves bends through at least 90.degree., the various strings bearing on one another and so accentuating the compression-abrasion problem.
The French Pat. No. FR-B-80 20663 describes solution to the problem of obtaining a composite tennis racket string, which consists in coating polyamide multifilament yarns with a solution of polyurethane which can be melted by the application of heat, the multifilament yarns being impregnated right through, dried, twisted and then heated so that the polyamide fibers form an armature between the strands of which there is disposed a flexible binder which contributes to a rapid return to the rest position after impact with the ball. Strings manufactured using this method have been found to offer playing qualities comparable with or better than those of the best natural yarns, with very much enhanced durability.
The methods which have just been described are nevertheless relatively slow by virtue of the need for prolonged heat treatment, whether this is to evaporate solvents followed by heating or a single melting operation. This results in a high manufacturing cost, in particular because of the thermal energy employed. In all these methods, it is necessary to heat the entire cord. The objective of the present invention is to remedy this disadvantage and to provide for faster manufacture and consequently a reduced manufacturing cost as compared with known methods.